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      1 /* Compile-time assert-like macros.
      2 
      3    Copyright (C) 2005-2006, 2009-2012 Free Software Foundation, Inc.
      4 
      5    This program is free software: you can redistribute it and/or modify
      6    it under the terms of the GNU General Public License as published by
      7    the Free Software Foundation; either version 3 of the License, or
      8    (at your option) any later version.
      9 
     10    This program is distributed in the hope that it will be useful,
     11    but WITHOUT ANY WARRANTY; without even the implied warranty of
     12    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
     13    GNU General Public License for more details.
     14 
     15    You should have received a copy of the GNU General Public License
     16    along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
     17 
     18 /* Written by Paul Eggert, Bruno Haible, and Jim Meyering.  */
     19 
     20 #ifndef _GL_VERIFY_H
     21 # define _GL_VERIFY_H
     22 
     23 
     24 /* Define _GL_HAVE__STATIC_ASSERT to 1 if _Static_assert works as per C11.
     25    This is supported by GCC 4.6.0 and later, in C mode, and its use
     26    here generates easier-to-read diagnostics when verify (R) fails.
     27 
     28    Define _GL_HAVE_STATIC_ASSERT to 1 if static_assert works as per C++11.
     29    This will likely be supported by future GCC versions, in C++ mode.
     30 
     31    Use this only with GCC.  If we were willing to slow 'configure'
     32    down we could also use it with other compilers, but since this
     33    affects only the quality of diagnostics, why bother?  */
     34 # if (4 < __GNUC__ || (__GNUC__ == 4 && 6 <= __GNUC_MINOR__)) && !defined __cplusplus
     35 #  define _GL_HAVE__STATIC_ASSERT 1
     36 # endif
     37 /* The condition (99 < __GNUC__) is temporary, until we know about the
     38    first G++ release that supports static_assert.  */
     39 # if (99 < __GNUC__) && defined __cplusplus
     40 #  define _GL_HAVE_STATIC_ASSERT 1
     41 # endif
     42 
     43 /* Each of these macros verifies that its argument R is nonzero.  To
     44    be portable, R should be an integer constant expression.  Unlike
     45    assert (R), there is no run-time overhead.
     46 
     47    If _Static_assert works, verify (R) uses it directly.  Similarly,
     48    _GL_VERIFY_TRUE works by packaging a _Static_assert inside a struct
     49    that is an operand of sizeof.
     50 
     51    The code below uses several ideas for C++ compilers, and for C
     52    compilers that do not support _Static_assert:
     53 
     54    * The first step is ((R) ? 1 : -1).  Given an expression R, of
     55      integral or boolean or floating-point type, this yields an
     56      expression of integral type, whose value is later verified to be
     57      constant and nonnegative.
     58 
     59    * Next this expression W is wrapped in a type
     60      struct _gl_verify_type {
     61        unsigned int _gl_verify_error_if_negative: W;
     62      }.
     63      If W is negative, this yields a compile-time error.  No compiler can
     64      deal with a bit-field of negative size.
     65 
     66      One might think that an array size check would have the same
     67      effect, that is, that the type struct { unsigned int dummy[W]; }
     68      would work as well.  However, inside a function, some compilers
     69      (such as C++ compilers and GNU C) allow local parameters and
     70      variables inside array size expressions.  With these compilers,
     71      an array size check would not properly diagnose this misuse of
     72      the verify macro:
     73 
     74        void function (int n) { verify (n < 0); }
     75 
     76    * For the verify macro, the struct _gl_verify_type will need to
     77      somehow be embedded into a declaration.  To be portable, this
     78      declaration must declare an object, a constant, a function, or a
     79      typedef name.  If the declared entity uses the type directly,
     80      such as in
     81 
     82        struct dummy {...};
     83        typedef struct {...} dummy;
     84        extern struct {...} *dummy;
     85        extern void dummy (struct {...} *);
     86        extern struct {...} *dummy (void);
     87 
     88      two uses of the verify macro would yield colliding declarations
     89      if the entity names are not disambiguated.  A workaround is to
     90      attach the current line number to the entity name:
     91 
     92        #define _GL_CONCAT0(x, y) x##y
     93        #define _GL_CONCAT(x, y) _GL_CONCAT0 (x, y)
     94        extern struct {...} * _GL_CONCAT (dummy, __LINE__);
     95 
     96      But this has the problem that two invocations of verify from
     97      within the same macro would collide, since the __LINE__ value
     98      would be the same for both invocations.  (The GCC __COUNTER__
     99      macro solves this problem, but is not portable.)
    100 
    101      A solution is to use the sizeof operator.  It yields a number,
    102      getting rid of the identity of the type.  Declarations like
    103 
    104        extern int dummy [sizeof (struct {...})];
    105        extern void dummy (int [sizeof (struct {...})]);
    106        extern int (*dummy (void)) [sizeof (struct {...})];
    107 
    108      can be repeated.
    109 
    110    * Should the implementation use a named struct or an unnamed struct?
    111      Which of the following alternatives can be used?
    112 
    113        extern int dummy [sizeof (struct {...})];
    114        extern int dummy [sizeof (struct _gl_verify_type {...})];
    115        extern void dummy (int [sizeof (struct {...})]);
    116        extern void dummy (int [sizeof (struct _gl_verify_type {...})]);
    117        extern int (*dummy (void)) [sizeof (struct {...})];
    118        extern int (*dummy (void)) [sizeof (struct _gl_verify_type {...})];
    119 
    120      In the second and sixth case, the struct type is exported to the
    121      outer scope; two such declarations therefore collide.  GCC warns
    122      about the first, third, and fourth cases.  So the only remaining
    123      possibility is the fifth case:
    124 
    125        extern int (*dummy (void)) [sizeof (struct {...})];
    126 
    127    * GCC warns about duplicate declarations of the dummy function if
    128      -Wredundant-decls is used.  GCC 4.3 and later have a builtin
    129      __COUNTER__ macro that can let us generate unique identifiers for
    130      each dummy function, to suppress this warning.
    131 
    132    * This implementation exploits the fact that older versions of GCC,
    133      which do not support _Static_assert, also do not warn about the
    134      last declaration mentioned above.
    135 
    136    * GCC warns if -Wnested-externs is enabled and verify() is used
    137      within a function body; but inside a function, you can always
    138      arrange to use verify_expr() instead.
    139 
    140    * In C++, any struct definition inside sizeof is invalid.
    141      Use a template type to work around the problem.  */
    142 
    143 /* Concatenate two preprocessor tokens.  */
    144 # define _GL_CONCAT(x, y) _GL_CONCAT0 (x, y)
    145 # define _GL_CONCAT0(x, y) x##y
    146 
    147 /* _GL_COUNTER is an integer, preferably one that changes each time we
    148    use it.  Use __COUNTER__ if it works, falling back on __LINE__
    149    otherwise.  __LINE__ isn't perfect, but it's better than a
    150    constant.  */
    151 # if defined __COUNTER__ && __COUNTER__ != __COUNTER__
    152 #  define _GL_COUNTER __COUNTER__
    153 # else
    154 #  define _GL_COUNTER __LINE__
    155 # endif
    156 
    157 /* Generate a symbol with the given prefix, making it unique if
    158    possible.  */
    159 # define _GL_GENSYM(prefix) _GL_CONCAT (prefix, _GL_COUNTER)
    160 
    161 /* Verify requirement R at compile-time, as an integer constant expression
    162    that returns 1.  If R is false, fail at compile-time, preferably
    163    with a diagnostic that includes the string-literal DIAGNOSTIC.  */
    164 
    165 # define _GL_VERIFY_TRUE(R, DIAGNOSTIC) \
    166     (!!sizeof (_GL_VERIFY_TYPE (R, DIAGNOSTIC)))
    167 
    168 # ifdef __cplusplus
    169 #  if !GNULIB_defined_struct__gl_verify_type
    170 template <int w>
    171   struct _gl_verify_type {
    172     unsigned int _gl_verify_error_if_negative: w;
    173   };
    174 #   define GNULIB_defined_struct__gl_verify_type 1
    175 #  endif
    176 #  define _GL_VERIFY_TYPE(R, DIAGNOSTIC) \
    177     _gl_verify_type<(R) ? 1 : -1>
    178 # elif defined _GL_HAVE__STATIC_ASSERT
    179 #  define _GL_VERIFY_TYPE(R, DIAGNOSTIC) \
    180      struct {                                   \
    181        _Static_assert (R, DIAGNOSTIC);          \
    182        int _gl_dummy;                          \
    183      }
    184 # else
    185 #  define _GL_VERIFY_TYPE(R, DIAGNOSTIC) \
    186      struct { unsigned int _gl_verify_error_if_negative: (R) ? 1 : -1; }
    187 # endif
    188 
    189 /* Verify requirement R at compile-time, as a declaration without a
    190    trailing ';'.  If R is false, fail at compile-time, preferably
    191    with a diagnostic that includes the string-literal DIAGNOSTIC.
    192 
    193    Unfortunately, unlike C11, this implementation must appear as an
    194    ordinary declaration, and cannot appear inside struct { ... }.  */
    195 
    196 # ifdef _GL_HAVE__STATIC_ASSERT
    197 #  define _GL_VERIFY _Static_assert
    198 # else
    199 #  define _GL_VERIFY(R, DIAGNOSTIC)				       \
    200      extern int (*_GL_GENSYM (_gl_verify_function) (void))	       \
    201        [_GL_VERIFY_TRUE (R, DIAGNOSTIC)]
    202 # endif
    203 
    204 /* _GL_STATIC_ASSERT_H is defined if this code is copied into assert.h.  */
    205 # ifdef _GL_STATIC_ASSERT_H
    206 #  if !defined _GL_HAVE__STATIC_ASSERT && !defined _Static_assert
    207 #   define _Static_assert(R, DIAGNOSTIC) _GL_VERIFY (R, DIAGNOSTIC)
    208 #  endif
    209 #  if !defined _GL_HAVE_STATIC_ASSERT && !defined static_assert
    210 #   define static_assert _Static_assert /* C11 requires this #define.  */
    211 #  endif
    212 # endif
    213 
    214 /* @assert.h omit start@  */
    215 
    216 /* Each of these macros verifies that its argument R is nonzero.  To
    217    be portable, R should be an integer constant expression.  Unlike
    218    assert (R), there is no run-time overhead.
    219 
    220    There are two macros, since no single macro can be used in all
    221    contexts in C.  verify_true (R) is for scalar contexts, including
    222    integer constant expression contexts.  verify (R) is for declaration
    223    contexts, e.g., the top level.  */
    224 
    225 /* Verify requirement R at compile-time, as an integer constant expression.
    226    Return 1.  This is equivalent to verify_expr (R, 1).
    227 
    228    verify_true is obsolescent; please use verify_expr instead.  */
    229 
    230 # define verify_true(R) _GL_VERIFY_TRUE (R, "verify_true (" #R ")")
    231 
    232 /* Verify requirement R at compile-time.  Return the value of the
    233    expression E.  */
    234 
    235 # define verify_expr(R, E) \
    236     (_GL_VERIFY_TRUE (R, "verify_expr (" #R ", " #E ")") ? (E) : (E))
    237 
    238 /* Verify requirement R at compile-time, as a declaration without a
    239    trailing ';'.  */
    240 
    241 # define verify(R) _GL_VERIFY (R, "verify (" #R ")")
    242 
    243 /* @assert.h omit end@  */
    244 
    245 #endif
    246